—I have never been to the Great wall. —If you have an opportunity there you must take it

问题补充：

—I have never been to the Great wall.

—If you have an opportunity there, you must take it.

A. to go

B. to going

C. going

D. go

答案：

(答案→)A. to go

B. to going

C. going

D. go

解析：句意：我从来没有到过长城。——如果你有一个到那儿的机会，你一定要抓住。动词不定式做目的状语表示具体的将要进行的动作，结合语境可知选A。